i tried ady...but i still cant find it....3rd equation got 2 unknown Ib and Vbe...i just dont know how to solve it without the type of transistor given...can you help me to figure it out?

Technically you are correct that you don't know Vbe, but there is an approximation you can make in this type of circuit. Assume Vbe is 0.6 V and you will get a decent answer.

The alternative is to use the Schokley diode law, but then you need more information about temperature and transistor characteristics. Given those uncertainties, assuming Vbe=0.6 should be good enough.

Teacher only taught me silicon transistor,Vbe=0.7 and germanium transistor,Vbe=0.3 ....but i dont know can assume one ^^? ......and in this question both type of transistor is not given that's why i dont know how to proceed..:(

Teacher only taught me silicon transistor,Vbe=0.7 and germanium transistor,Vbe=0.3 ....but i dont know can assume one ^^? ......and in this question both type of transistor is not given that's why i dont know how to proceed..:(

Well, you are making good points that technically the problem does not specify the transistor. But, this approach is not going to let you solve the problam.

Traditionally, if the transistor is not specified, one can assume it is a silicon device, and 0.6 or 0.7 V can be used for Vbe.

Yes, you can find an approximate answer based on assumptions, but it amounts to the same thing. Any reasonable assumptions will put Vbe in the 0.6 V to 0.7 V range and the answer is not very sensitive to changes to Vbe when Vbb is 3 V or greater.

There is going to be sensitivity to changes in temperature and transistor characteristics and part of the challenge to designing transistor circuits is making circuits that are less sensitive. You will be learning about this soon.

As long as the question does not provide the type of transistor,then we just assume it to be Vbe=0.7 ...am i right? i meant other than this question also?....

Typically, yes, but there are always exceptions to any rule. The basic idea in most circuit design is to make robust circuits that are not sensitive to temperature and transistor characteristics. The end result of this is that usually changes in Vbe are not important and the above approximation gives reasonable answers.

As long as the question does not provide the type of transistor,then we just assume it to be Vbe=0.7 ...am i right? i meant other than this question also?....

stevenb gave a good answer. Also, you can think of the base-to-emitter path as similar to a diode. As long as there is something providing voltage between the base and emitter, it will (usually) be about 0.7V.